As a reminder, the acidic pH of the skin preserves the integrity and cohesion of the stratum corneum and protects the skin from microbial infections. Indeed, when the skin's pH increases, the enzymes responsible for the production of ceramides, which have an optimal acidic pH, are deactivated, compromising the structure and function of the stratum corneum.
Furthermore, while the enzymes synthesising lipids decrease, other enzymes increase at acidic pH: the serine proteases. These lead to a reduction in corneodesmosomes, through the degradation of desmoglein-1, which play a role in ensuring the cohesion of the elements of the stratum corneum.
When the pH of the skin is deregulated, the skin no longer resists external aggressions and fails to retain its hydration. This is conducive to the development of dermatoses such as atopic dermatitis. Indeed, it has been shown that the higher the pH, the more intense the itching and the greater the dryness.
Beyond the fact that in atopic dermatitis the structure of the stratum corneum is altered, colonisation by Staphylococcus aureus is a common characteristic of affected patients and is considered a major pathogenic factor in atopic dermatitis. However, the adhesion of Staphylococcus aureus to human keratinocytes increases with the rise in pH.
The growth of protective bacteria in the skin microbiota is optimal at acidic pH levels, whereas pathogenic bacteria thrive at neutral and basic pH levels (Staphylococcus aureus, Cutibacterium acnes). Therefore, a disruption in skin pH disturbs the skin's microbiota and hinders anti-microbial protection. The risk of infections is thus increased. Moreover, serine proteases, which are activated by a high pH, trigger the synthesis of cytokines that cause inflammation.